The present invention relates to a mass flow controlled liquid vaporizing/feeding method of vaporizing liquid feedstock and feeding the resultant gas at an accurate mass flow to a succeeding process, and to a liquid vaporizer/feeder utilizing the method.
A mass flow controlled liquid vaporizing/feeding method, where liquid feedstock is vaporized and the resultant gas is fed at an accurate mass flow to a succeeding process, and a liquid vaporizer/feeder utilizing the method are used in various industrial fields. We herein will explain a liquid vaporizing/feeding method and a liquid vaporizer/feeder utilizing the method in exemplary situations where they are applied to a semiconductor fabricating process, and particularly, to an atmospheric pressure CVD apparatus for forming thin film on a surface of a wafer. It should be noted that the present invention is not to be applied only to the semiconductor fabricating process.
Although feedstock furnished to a CVD apparatus or process has conventionally been a gaseous matter, semiconductor gas has adverse attributes such as explosiveness, virulent poisonousness, corrosiveness, etc., and hence, in recent years liquid feedstock has been gradually used in view of safety and allowing for manufacturing thin films of excellent properties.
Such gaseous feedstock includes, for example, TEOS (Tetra Ethyl Ortho Silicate). A bubbling system as shown in FIG. 8 is employed as a feeding system of TEOS to an atmospheric pressure CVD apparatus; however, it has various disadvantages as mentioned later and is already difficult to apply to the next generation devices of more enhanced integration.
FIG. 8 depicts a liquid vaporizing/feeding system utilizing the prior art bubbling mechanism for the semiconductor fabricating process, and the disadvantage of the system will be described below:
In FIG. 8, there are depicted a liquid mass flow controller MFC, liquid feedstock L, tank-like feedstock reservoir T, mass flowmeter MFM, and a furnace (e.g., atmospheric pressure CVD apparatus). Also depicted are an output flow rate Vc of a carrier gas C, an output flow rate Vv of a feedstock gas, a mass flow rate Qc of the carrier gas discharged from the mass flow controller, and a mass flow rate Qv of the feedstock gas.
Referring to FIG. 8, when the mass flow controlled carrier gas C expressed by the flow rate Qc is injected into the reservoir filled with the liquid feedstock L to pressurize it, bubbles of the carrier gas C rise in the liquid feedstock L. Part of the liquid feedstock L is vaporized under the vapor pressure of the liquid feedstock L and flows out of the reservoir T, mixed with the carrier gas C. The mass of vapor-gas mixture manufactured in this manner (referred to as "mixed gas MG" hereinafter) is represented by (Qc+Qv). Mass flow of the mixed gas MG is measured by the mass flowmeter MFM. A measure obtained is output as (Vc+Vv) by the mass flowmeter MFM. The output flow rate Vv of only the liquid feedstock L can be obtained by subtracting the output value Vc of only the carrier gas C from the output value (Vc+Vv). If the resultant output value is fed back to the mass flow controller MFC and controlled therein, a constant mass flow of the feedstock gas can be continuously fed, and the mass flow can be set to any desirable value as required.
However, although the feeding system of the liquid feedstock L utilizing the above-mentioned bubbling mechanism can be used when a ratio of the feedstock gas of the liquid feedstock L to the carrier gas C sensed by the mass flowmeter MFM is relatively large, it often cannot actually be employed because of an extremely bad accuracy when the ratio is small.
Additionally, since it is very difficult to determine conditions of the bubbling, a good accuracy cannot be expected even in the case where the bubbling mechanism can be applied, and there is almost no way of applying the feeding system to the expected next generation semiconductors in the near future.
An applicable range of the bubbling capacity is very small, unadjustable from the desired minimum level to the maximum. Moreover, there are various disadvantages; for example, a mist caused by the bubbling flows into the mass flowmeter MFM along with the carrier gas C and is gathered therein to cause an error or cause malfunction.
Accordingly, it is an object of the present invention to perfectly vaporize liquid feedstock and feed feedstock gas continuously at an extremely high accuracy of mass flow with adjustability of a wide range of vaporizing/feeding capacity from the desired minimum level to the maximum, so as to be satisfactory for manufacturing the next generation semiconductors.